1. Field of the Invention
This invention relates to digital video recording. More particularly, it relates to methods of recording and reproducing digital video signals on and from a magnetic tape, to magnetic tapes having signals recorded thereon by such methods, and to apparatus for recording and reproducing signals by such methods.
2. Description of the Prior Art
The practice of digitally recording video signals and the associated audio signals is well established and is in common use at least in broadcasting studios. For the purpose of such recording, the incoming analog video and audio signals are sampled, the resulting samples are pulse code modulation coded, and the resulting digital signals, usually after further coding to provide for error detection and correction after reproduction, are recorded in oblique tracks on a magnetic tape using a digital video tape recorder. Digital audio signals are usually grouped in one or two blocks which are located at the beginning and/or the end of each oblique track.
One form of digital video tape recorder which has been proposed records twelve oblique tracks per field when the signal recorded relates to a 625 lines per frame, 50 fields per second television system, and ten oblique tracks per field when the signal recorded relates to a 525 lines per frame, 60 fields per second television system. This can be achieved by providing the rotary head drum of the digital video tape recorder with four recording heads arranged in two pairs on diametrically opposite sides of the rotary head drum, which is arranged to be rotated at three times the 50 fields per second field rate or 2.5 times the 60 fields per second field rate, that is, in each case, at 150 revolutions per second.
To improve the protection against errors and in particular to provide protection against a situation in which on reproduction the output of one of the four reproducing heads is lost entirely, it is preferred to distribute the data evenly between the four recording heads when recording. In 625/50 operation this presents no problems, because a field occupies twelve oblique tracks and twelve is divisible by four. In 525/60 operation a field occupies ten oblique tracks, and to overcome the problem resulting from ten not being divisible by four, it has been proposed to consider the ten oblique tracks as twenty half-tracks and to start recording of each field at a mid-track position. In other words, the field boundaries are at mid-track positions. This is illustrated in FIG. 1 of the accompanying drawings, in which FIG. 1A indicates diagrammatically the format of twelve oblique tracks making up one field in 625/50 operation, the letters a, b, c and d indicating the heads which record and reproduce the respective tracks, and FIG. 1B indicates diagrammatically the format of ten oblique tracks, or more precisely twenty half-tracks, making up one filed in 525/60 operation.
These formats permit the required even distribution of the data between the four heads, but results is a problem when reproducing in 525/60 operation at speeds different from the normal reproduction speed, particularly in fast motion reproduction. So-called dynamic tracking is now well known. This technique involves mounting each reproducing head, or in the case of the four-head digital video tape recorder referred to above, each pair of reproducing heads, on a respective controllable element such as a bimorph leaf. Then, by applying suitable control signals to the controllable elements, the positions of the reproducing heads can be varied relative to the oblique tracks to be scanned, so as more accurately to follow an oblique track and to enable the head to be jumped from one oblique track to another during the interval when the reproducing head is out of contact with the magnetic tape.
In fast motion reproduction it would be convenient to be able to omit complete fields during reproduction, but to jump even just one field the dynamic tracking arrangement must effect a quite substantial movement of the associated head pair. This is possible in 625/50 operation because the large movement takes place during the interval when the head pair is out of contact with the magnetic tape, but in 525/60 operation this is not possible because a field finishes at the mid-track position as indicated in FIG. 1B and there is therefore very little time available to make a field jump.